You can always invent your own. It's not like "a blessing of unicorns" is some ancient phrase; somebody just thought it sounded right and used it, and it got popular. Most of these obscure terms for groups of things are just jokes anyway.
– sumelicApr 20 '15 at 18:58

2

Sounds like the OP has 99 problems, but a lich ain't one.
– MT_HeadApr 20 '15 at 20:45

3 Answers
3

I don't think there is an established collective noun for this fictional creature. Unicorns have somewhat an established collective noun because unicorn is a well-known legendary/fictional animal. Blessing can fall under terms of venery and these terms are more prevalent when we are more familiar with the animal and if they are used more frequently in literature.

Having said that, one can use their imagination to coin a collective noun for liches and it would be more suitable if fiction writers do it. You can also use different collective nouns depending on the context.

For example, One of the writers used scores:

The demon waddled into the main hall on its short legs. There were scores of liches.

I think "score" here is used in its literal sense, meaning "twenty" in the same way one might say, "dozens of soldiers." It's not a collective noun. Similarly, the "chorus" is probably imagery meant to describe how they are acting together, like an "army of street sweepers," not a collective noun. A pod of whales, a parliament of owls, or a murder of crows doesn't function like a pod of peas, the Parliament of England, or the murder of Jesse James.
– jejorda2Apr 20 '15 at 20:11

The Book of St. Albans, printed 1486, offered names for 165 different animal groups, including a gaggle of women, and a diligence of messengers, and influenced the Standard English Lexicon through Gervase Markham's The Gentleman's Academic in 1595.

Modern group names often reflect an essential quality of the creatures, for example:

a flight of butterflies

an intrusion of cockroaches

a bask of crocodiles

a gaggle of geese

a pride of lions

a sneak of weasels

Wikipedia.org

Naming groups of animals is fun, and naming sci-fi creatures should be just as fun:

Liches, by the nature of the person who usually succeed in becoming them, tend to be smart, cunning, ambitious and treacherous.
So, the only time I could see a group of them working together it would really be a sort of "shake your hand while trying to stab you in the back" proposition for all involved. So, a cabal maybe?

Alternatively, since there is no specific term for a group of them that I know of, you could just use the term for a group of undead creatures, since they fit that decision, and there are a lot of different ones. The most fitting of those that I can think of would be a "scourge".